1. Field of the Invention
The present invention relates to a method for the injection molding of a resin. More particularly, the present invention is concerned with a method for the injection molding of a resin to produce a shaped resin article having a thick portion biasedly and having an improved surface finish. In the method of the present invention, an excess filling of the cavity of a mold is effected to form a molten resin mass having opposite, first and second surfaces respectively facing opposite, first and second inner walls of the mold cavity, the mold having a specific structure in which a gas-release passage is formed in association with the mold cavity and with a mating surface of the mold, wherein the gas-release passage has an inner opening in an inner wall surface of the mold cavity and communicates with the outside of the mold, and a pressurized gas is introduced into the mold cavity on a side of the first surface of the molten resin mass, thereby forming a closed space which is filled with the pressurized gas and separated from the gas-release passage through the molten resin mass, so that the pressurized gas filled in the closed space urges the second surface of the molten resin mass to be pressed against the second inner wall of the mold cavity. The injection molding method of the present invention is advantageous not only in that the occurrence of a sink mark on a front surface of a shaped resin article can be prevented without causing the problem of the occurrence of burrs (flashes) thereon, but also in that there is no need for use of a completely gas-tightly sealed mold which requires a high equipment cost and a high maintenance cost, and that an injection molding can be performed with a relatively low excess-filling ratio, as compared to the conventional excess-filling injection molding method.
2. Prior Art
It is widely known that when a shaped resin article which has a relatively large thickness or has a locally protruded portion on a back surface thereof is produced by injection molding a molten resin, a dent, i.e., the so-called "sink mark", is likely to occur on a front surface of the shaped resin article in accordance with the volume shrinkage of the molten resin during the cooling thereof. For preventing the occurrence of a sink mark on a front surface of a shaped resin article, a method has conventionally been known, wherein when an injection pressure is applied, an increased level of pressure is used, and, after the injection, the application of pressure is continued while cooling the injected molten resin (this method is hereinafter referred to as "resin pressure method".). In this resin pressure method, the applied injection pressure is generally in the range from 50 to 200 MPa in terms of the maximum resin pressure in the mold cavity.
However, as described in Japanese Patent Application Laid-Open Specification No. 50-75247, the above-mentioned resin pressure method has problems. For example, it is difficult to appropriately apply pressures in two stages so as to achieve the effects aimed at by the resin pressure method, so that the molding operation tends to become cumbersome and time-consuming. Furthermore, in the resin pressure method, the shaped resin articles produced are likely to have burrs formed along a parting line of the mold. Therefore, the removal of the burrs is necessary, thereby inevitably causing a problem such that additional work or steps are necessary in a molding operation. On the other hand, when a relatively low pressure is employed in the resin pressure method in order to prevent the occurrence of burrs, sink marks are likely to occur at a portion of a shaped resin article, which is positioned away from a gate in a mold cavity, that is, at a portion on which a satisfactory resin pressure cannot be obtained.
For solving the problems of the resin pressure method, the above-mentioned Japanese Patent Application Laid-Open Specification No. 50-75247 proposes a gas-assisted injection molding method in which a mold is used which comprises a fixed mold half and a movable mold half with its inner wall having a core protruding therefrom toward the inner wall of the fixed mold half, so that a concave mold cavity defined by the fixed mold half and the movable mold half having the core portion is provided. The core has a hole extending in the direction of the longitudinal axis thereof, and the hole has a poppet valve disposed at the top thereof. In this method, after a molten resin has been injected into a mold cavity, the poppet valve is operated to push up the molten resin mass, so that a space is produced between the core and the inner surface of the concave molten resin mass formed in the concave mold cavity, and then a pressurized gas is injected through the poppet valve into the above-mentioned space, thereby pressing the outer surface of the molten resin mass against the cavity inner wall remote from the core. This gas-assisted injection molding method is intended to prevent the occurrence of sink marks by using a pressurized gas, instead of using the two-stage application of pressure as in the above-mentioned resin pressure method which, as described above, is likely to be unfavorably accompanied by the occurrence of burrs along a parting line of a mold cavity. In Japanese Patent Application Laid-Open Specification No. 50-75247, there is no description about the amount of molten resin to be injected.
International Patent Application Publication No. WO 90/06220 (corresponding to U.S. Pat. No. 5,273,707 and EP No. 400135 ) discloses a gas-assisted injection molding method in which a molten resin is injected into the mold cavity in a volume smaller than the volume of the mold cavity, specifically in a volume of 90% to 95% of the volume of the mold cavity, and then a pressurized gas is introduced into the remaining space in the cavity. However, a gap is likely to be formed between the inner wall (remote from the side of the gas introduction) of the mold cavity and the molten resin injected into the mold cavity. Hence, the gas intrudes into the gap, causing the occurrence of sink marks.
Further, the gas which had intruded into the gap is likely to escape from the mold cavity through a parting line of the mold cavity, so that the effect of the pressurized gas becomes insufficient to prevent the occurrence of sink marks.
As is apparent from the above, in the conventional gas-assisted injection molding methods, for preventing the occurrence of burrs and for assuring a space in a mold cavity into which a pressurized gas is to be introduced, a molten resin is injected into a mold cavity in a volume equal to or smaller than the volume of the mold cavity.
International Patent Application Publication No. WO 93/14918 discloses the use of a mold having a sealed construction in which not only a parting line between a movable mold half and a fixed mold half, but also a space between an ejector pin and an inner wall of an ejector pin slide hole provided in the movable mold half, is sealed. In addition, this patent document teaches that a weir formed of a protrusion (such as a protrusion having a triangular cross-section) is provided around a portion of the inner wall of a mold cavity on which it is intended to receive a gas pressure, so that a gas pressure can be effectively applied to a desired portion of an injected molten resin for preventing the occurrence of a sink mark.
However, with the use of the mold having a sealed construction, it is impossible to prevent an unfavorable intrusion of a pressurized gas into a portion of the mold cavity on a side of the surface of an injected molten resin mass which surface should be pressed against a cavity inner wall facing it. Therefore, close contact between the above-mentioned surface of the molten resin mass and the cavity inner wall facing it cannot be achieved, resulting in an unsatisfactory surface finish of the resultant shaped resin article. In addition, since a weir is provided on the inner wall of the mold cavity, the freedom of design of a shaped article to be produced is inevitably limited. Further, it is noted that in International Patent Application Publication No. WO 93/14918, it is described that the mold cavity is filled or nearly filled with a molten resin.
International Patent Application Publication No. WO 96/02379 discloses a method for the non-resin fluid-assisted injection molding of a resin to produce a shaped resin article, wherein an excess filling of a mold cavity of a sealed mold with a molten resin is effected in a specific excess filling ratio [corresponding to an excess ratio of a resin by an amount corresponding to 30 to 90% of a difference between the volume of the mold cavity and a volume exhibited by the resin in accordance with a shrinkage thereof when the molten resin filled in the mold cavity is cooled to room temperature], to thereby form a molten resin mass having opposite, first and second surfaces respectively facing opposite inner walls of the mold cavity, and a pressurized non-resin fluid is subsequently introduced into the mold cavity on a side of the first surface of the molten resin mass to thereby press the second surface of the molten resin mass against the cavity inner wall remote from the side of the introduction of the non-resin fluid. However, in this technique, it is requisite to use a mold having a tightly sealed construction, so that it becomes necessary to provide a sealed chamber in the mold to thereby seal a space between an ejector pin and an inner wall of an ejector pin slide hole provided in a movable mold half. Therefore, this technique has problems such that a mold having a complicated structure is required, and that for expelling the air or the gas out of the mold to transfer the air or the gas into the sealed chamber, it is necessary to inject a resin into the mold cavity in a relatively large excess amount or to inject a resin under high pressure.
With respect to the above-mentioned conventional molding methods using a pressurized gas (or non-resin fluid), there have been a tendency that the amount of the gas introduced into the mold cavity is controlled to be as small as possible, so as to render efficient the operation of the introduction of the gas, and a tendency that a mold having a tightly sealed construction is used, so as to prevent the escape of the gas from the mold cavity.
Recently, large-size shaped articles, such as automobile parts and casings of home electric appliances, have been in increasing demand. In order that the production cost may be reduced by rendering it unnecessary to conduct finishing works, such as painting and plating, it has been desired to produce a shaped resin article having an excellent surface finish, in which a sink mark is suppressed to a visually unnoticeable level and which exhibits excellent and uniform gloss and luster.
However, as described hereinabove, in the conventional injection molding methods, when a shaped resin article to be produced has a locally protruded portion on a back surface thereof at which the article has an increased thickness, the occurrence of a sink mark defect on a front surface thereof at a portion corresponding to the locally protruded portion cannot be satisfactorily suppressed without causing the problem of the occurrence of burrs.